1. Field of the Invention
The present invention relates to an optical communication system and, more particularly, to a coherent optical communication system.
2. Description of the Related Art
Fiber optic technology has completely penetrated the long-haul telephony network due to its low loss and high bandwidth. In the area of local loop applications, financially attractive options have developed more slowly. Substantial research effort has been directed towards developing technology to implement fiber optics within local loop applications (e.g., fiber in the local loop). However, cost, capacity, and switching problems associated with implementing fiber optics in local loop applications still must be overcome.
Recently, technologies have developed in an effort to more cost effectively introduce fiber into the local loop. For example, a passive optical network (PON) is an optical transmission system requiring no active components to direct optical signals between a central office (CO) or host digital terminal, and a network subscriber's terminal equipment. PONs typically embody a first star formed of a plurality of optical fibers which extend from a CO to each of a plurality of remote nodes. Each remote node may be envisioned as central to a second star formed of a second plurality of optical fibers extending from the remote node, each to one of a plurality of optical network units (ONU). Two well known PON architectures considered for deploying optical fibers into the local loop are "Telephone Over Passive Optical Networks" (TPON) and "Passive Photonic Loops" (PPL).
In the TPON architecture, the CO broadcasts a common signal to all end users. Information is segregated within the broadcast signal in individual time slots as a time division multiplexed (TDM) signal and/or sub-carrier multiplexed channels. A star coupler at the remote node distributes the broadcast signals to the optical network units. Upstream information is usually transmitted from each ONU within a particular time slot, received at the remote node, multiplexed and directed to the CO. Management of collisions in time and trade off between delivered optical power and the number of end users limits upgradability and deployment of conventional TPON architecture.
In the PPL architecture, each ONU is assigned a unique wavelength and optical information is wavelength segregated within a transmitted signal. In a wavelength division multiplexing (WDM) PON scheme, the CO assigns each ONU a unique wavelength. Optical information is transmitted from the CO to one of a plurality of remote nodes according to wavelength. Each remote node optically demultiplexes its received signals, and directs the demultiplexed signals by wavelength to each ONU. For upstream transmission, each ONU includes a separate optical transmitter at the ONU's assigned wavelength. Each ONU transmits signals to the remote node where the signals are incorporated into a composite signal and transferred to the CO. While WDM PON's have excellent power budgets in general, because all the light intended for a subscriber is directed to that subscriber and vice versa, implementation of WDM PON's is quite costly. For instance, the subscriber must have a wavelength-specific laser at the ONU.
In an effort to reduce the cost of implementing WDM PONs for fiber in loop applications and ameliorate operations, a Communication System Based on Remote Interrogation of Terminal Equipment (RITE-Net.RTM.) has been developed and is disclosed in U.S. Pat. No. 5,559,624 to Darcie et al. ("the '624 patent") and is incorporated herein by reference. In the '624 patent, a CO transmits an optical signal, which is modulated with downstream information, to a subscriber's ONU over a wavelength-division multiplexed network. A fraction of this downstream optical signal is detected in the ONU for recovery of the downstream information and the remainder is remodulated with the ONU's upstream information and returned to the CO. As such, the system disclosed in the '624 patent avoids the need of the wavelength-specific optical sources at each ONU. The RITE-Net.RTM. system thus lowers the cost for equipment required at each ONU. In addition to the RITE-Net.RTM. system providing WDM performance potential at reduced cost, the RITE-Net.RTM. system is flexible to allow additional revenue to be produced when it is incorporated into an existing system.
Although the RITE-Net.RTM. architecture provides many distinct advantages, at some point in time the need for capacity may become so great that even dedicated lasers with several hundred Mb/s capabilities may be inadequate. At that point, the intrinsic power limitation of having the central office laser supply light for both downstream and upstream signals may constrain the network's capacity. Accordingly, an enhanced optical receiver is needed at the CO to improve the upstream loss budget.
In general, coherent optical receivers include an optical local oscillator laser which is locked to the incoming signal from the remote transmitter, so that a square law photodetector can be utilized in a heterodyne or homodyne mode. In the heterodyne mode, the wavelength of the optical local oscillator laser is separated from the wavelength of the remote transmitter to create a beat frequency in the receiver. In the homodyne mode, the local oscillator is phase-locked to the incoming carrier wave at the same optical wavelength. In both cases, the reference signal from the local oscillator combines with the incoming optical signal at the photodetector surface, and the detector produces a current which is proportional to the product of the two optical signals. In particular, the carrier photocurrent depends linearly on the optical signal field and is effectively amplified by a factor proportional to the electric field produced by the local oscillator. Coherent optical fiber systems have the potential to greatly improve receiver sensitivity and selectivity. These potential increases provided by a coherent receiver could permit more channels to be transmitted on carriers closely spaced in optical frequency, thereby increasing capacity. A disadvantage of coherent optical systems is the necessity of acquiring the received carrier frequency to provide the correct local oscillator frequency for demodulating the received signal because determining, creating and locking the local oscillator frequency is difficult and costly to implement.
Accordingly, an optical communication system is needed which provides the benefits of coherent systems and alleviates the drawbacks.